Two thermal load capacity calculation methods including the water surface temperature method and the uniform mixing method for small lakes utilized as heat source-sink of surface water heat pump systems were introduced. The calculation results of the Hangzhou City indicate that the water surface temperature method is more convincing because it objectively reflects the process of discharge-mixing and water temperature recovering. Thermal load capacity calculations of small lakes in seven representative cities were performed. The results show that the thermal load capacity in summer varies from 23.4 W/m 2 to 29.5 W/m 2, and that in winter varies from 31.6 W/m 2 to 45.7 W/m 2. The key issue for the design of such surface water heat pump systems is to satisfy the constraint of thermal load capacity in summer.

A mathematical model for closed-loop lake water heat pump systems was developed. Dynamic simulation for a closed-loop lake water heat pump system in Changsha was performed according to the hourly load and the hourly weather data. The impacts of the lake depth and lake area on the entering fluid temperatures were analyzed, respectively, through the simulation. Simulation results show that the increase of lake depth would be favorable for the performance in cooling mode, but unfavorable for the performance in heating mode if all the other conditions remained unchanged. The increase of lake area would be more favorable for the performance in heating mode than for the performance in cooling mode based on the premise that all the other conditions remained unchanged. Antifreeze would be unnecessary for some cities whose weather condition is similar to Changsha as long as the heat exchanger size was adequate.